1. Field of the Invention
This invention relates to the production of fiber-reinforced resin stock. More particularly, the invention relates to the pultrusion of fiber-reinforced stock with a low viscosity, thermoplastic resin. The invention further relates to an improved pultrusion die for making sheets of fiber-reinforced stock. The apparatus and method of this invention produce thermoplastic stock without having to draw the fiber reinforcements through a resin bath positioned upstream from the die.
2. Technology Review
It is generally known to reinforce materials by imbedding fibrous strands into a matrix. This is especially true for matrices comprised of thermosetting resins. In some instances, reinforcing fibers are cut into short lengths before being formed into a composite by pressure molding on an item-by-item basis. British Patent No. 1,228,573 teaches making a composite by positioning carbon filaments, from 1 mm to 1 meter in length, between layers of thermoplastic (TP) film. The filaments may be randomly ordered or laid in the form of fabric between adjacent layers of TP film. After orientation of the filaments, these materials are heated in a compression mold to make sheet, rod or block products.
The batch processing of fiber-reinforced resin stock is not very economical from a production standpoint. The performance levels of such batch products may differ due to inconsistencies in wetting the short fibers added to such composites for reinforcement purposes.
The pultrusion of thermosetting resin products is generally well known. Therein, fiberglass or carbon rovings are pulled from a creel and soaked in a liquid resin bath before being cured in the heating zone of a pultrusion die. Gripper means pull on cooled, pultruded product to advance additional roving through the resin bath and into the die.
The pultrusion of certain thermoplastic resins is more complicated. High viscosity thermoplastics produce inconsistent wet-outs of the rovings passing through their molten resin tanks. Thermoplastics also exhibit a greater tendency to degrade when held at or above their crystallization temperatures for too long. As they degrade, low viscosity TP resins undergo crosslinking reactions to become higher in molecular weight and viscosity. Thermal degradation can be minimized by passing fiber reinforcements through a fluidized bed of resin powders. Powder processing still leads to wetting problems and inconsistencies in fiber penetration.